[shivanshdevops]

@SuperQ @discordianfish

Fixes #3052

Problem

The textfile collector merges metric families with the same name across .prom files.
When only some of those files carry a # HELP line, the families without one were given the generated Metric read from <files> help text.
That text differs from the explicit one, so the registry treats the families as inconsistent and drops part of the metric:

collected metric events_total ... has help "Metric read from a.prom, b.prom, c.prom" but should have "A nice help message."

With a.prom (no HELP), b.prom (HELP) and c.prom (no HELP), the series from b.prom, the only file that documents the metric, disappear from the output, and node_textfile_scrape_error stays 0, so nothing signals the loss.

While reproducing this I also hit a nil pointer dereference in the same check.
Once two files had given conflicting help texts for a metric, the condition mf.Help != nil && helpTexts[0] != *mf.Help || helpTexts[1] != "" evaluates to true for a third file without a help text, and the branch then dereferences mf.Help:

prometheus collector panic recovered: ... runtime error: invalid memory address or nil pointer dereference

client_golang recovers the panic, but every metric from the textfile collector is lost for that scrape.

Changes

• Only families that have a help text take part in the inconsistency check, since a family without one cannot conflict.
  This removes the nil dereference; the existing behaviour for conflicting explicit help texts (keep the first, drop the others, set node_textfile_scrape_error to 1) is unchanged.
• Before falling back to the generated Metric read from ... text, families without a help text now take the explicit help text recorded for the same metric.
  The generated text is still used when no file provides one.

Tests

Two fixture-driven cases are added to the existing TestTextfileCollector table:

• metrics_merge_partial_help: HELP only in the middle file; all six series are exported under the explicit help text and node_textfile_scrape_error is 0.
  On master the b.prom series are missing.
• metrics_merge_different_and_no_help: two conflicting help texts plus a file without one; the first help text wins, the conflicting file is dropped, the file without help is merged, and node_textfile_scrape_error is 1.
  On master this panics.

Verified with:

• make test
• GOOS=linux golangci-lint run ./... (v2.14.0, from make print-golangci-lint-version): 0 issues
• GOOS=linux go vet ./...